In a radio communication system, phase shift keying (PSK) signals, such as pi/4-QPSK, are designed to be constant amplitude signaling schemes. However, pulse shaping at a transmitter end, in order to minimize spreading of the signal into adjacent channels, destroys the constant amplitude property. Hence, the amplitude of the demodulated output signal varies. A radio communication system using such phase shift keying (PSK) modulation schemes must demodulate incoming information signals in a receiver. When such incoming information signals are noncoherent, or asynchronous, demodulation of the information signal can be achieved by a standard FM detector provided that the amplitude variation of the signals are eliminated or appropriately controlled. In other words, when a phase modulated signal is demodulated, the output is not compensated for amplitude variance. Thus, the demodulated output of a phase modulated signal is offset by the amplitude variance of the input signal. In conventional FM receivers a limiter performs the function of suppressing or eliminating the amplitude variance of the input signal. If a limiter is not used some other means must be used.
One method of eliminating the amplitude variation is to ascertain the magnitude of the amplitude variation and apply it to the amplitude varying demodulated signal in such a fashion that the amplitude variation is minimized or eliminated. Such a circuit has been proposed by John H. Park in his paper rifled "A FM Detector for Low S/N" published in the IEEE Transactions on Communication Technology, Vol. Com-18, No. 2, April 1970. FIG. 1 shows a block diagram of Park's circuit. The circuit assumes that the radio system can provide quadrature information signals 10, 12 to a demodulator 14. The demodulator 14 outputs an amplitude varying signal 15 that must be compensated in order to correctly decode the information signal. A sum-of-squares amplitude detector circuit 16 ascertains the magnitude of the amplitude variation of the signals. A divider circuit 18 uses the amplitude varying signal as one input and a direct current (DC) level proportional to the amplitude variation as a second input 17 and compensates the demodulator output 15. Traditionally, a divider circuit is built using an operational amplifier with a multiplier in its feedback loop. This method has practical implementation problems of balancing the feedback loop stability and providing adequate loop gain to achieve desired compensation.
Thus far, the best known method for achieving amplitude compensation is by dividing an amplitude varying signal by an appropriate compensating factor. The method of division in an analog circuit implementation, however, has many problems. Most importantly is the stability versus bandwidth problems associated with the feedback loop needed with the analog division circuit. A simple method of noncoherent demodulation designed to accomplish amplitude compensation needs to be developed that does not involve the standard division technique.